Optical Scanners
Many patents teach the art of transferring optical data to or from a surface in order to visualize or store that data in other forms, for example, as digital data stored on magnetic media. This art is realized in the form of optical scanners, which term generically means input devices which transfer data from a surface to a data store, output devices which transfer data from a data store to a photosensitive surface, and hybrid devices which are capable of performing either function. Common to all such devices are a number of functionalities which are realized in a variety of ways: an illumination system is employed to expose the photosensitive surface or to provide input to the scanner photodetective area, a means for relating the photosignal to position on the surface is provided, a means for controlling and/or processing the conversion of photosignal to or from the data store is provided, and a means for controlling the operation of the scanner system and mechanisms is provided.
One application of this technology is to the manufacture of artwork for the production of printed circuit boards. Typically printed circuits are produced when a copper plated substrate board is etched to produce a pattern of copper "traces", "pads" and other areas which are used, for example, to interconnect integrated circuits and other electronic components mounted on the board. Typically, a fully plated board is coated with photoresist; a mask of the desired pattern is placed in contact with the photoresist; the photoresist is flooded with ultraviolet light through the mask; the photoresist is developed and washed off. This process leaves developed photoresist in the desired pattern which protects the underlying copper during an etching process. The board is then etched to remove the unwanted copper, cleaned of photoresist, drilled as necessary to permit mounting of components, and otherwise processed in accordance with the art.
This invention teaches improved methods of producing the mask.
In one approach, specialized stable line art photographic film such as Kodak LP-7 or Dupont PCP-7 is exposed to produce the mask. Line art film responds to exposure very non-linearly; when properly exposed and processed it stores optical data in a binary fashion. Such film is either fully exposed (saturated black) or essentially transparent; edges of exposed area or lines are also very sharp and clearly defined. The pattern of fine lines and other artifacts required for a printed circuit photomask can be reproduced very well, so are any undesired artifacts imposed on the source data by the system employed to convert the stored data first to optical exposure signals and then to film image.
Throughout this application, when applied optical scanner images or data, the term "appearance" means close and faithful transfer of optical data to or from the data store. Gross distortions are not present in optical scanners at the edge of the current state-of-the-art, but small scale and intrusive artifacts are common: "streaking" and "banding" with grey scale reproduction and jagged eges or moire' with either line art or continuous tone reproduction. The first two artifacts are most intrusive when caused by spatially periodic errors in the positioning mechanism or temporarily periodic excursions in the illumination, typical causes are mechanical vibrations and lead screw errors. The last two artifacts are most commonly caused by digital data aliasing.
Even stable photographic film, such as that employed to produce printed circuit photomasks, is too sensitive to environmental and processing conditions easily to permit production of high accuracy printed circuit boards.
Photographic film dimensions vary with temperature, humidity and history. Controlling these variations is a major problem for the manufacturers of printed circuit boards and printed circuit phototools (the trade term for photomasks).
Under the current state-of-the-art for high precision photomasks, it is, in many cases, more practical to employ stable glass plates as the substrate instead of photographic film. This procedure is very much more expensive for producing phototools than employing photographic film, but controlling process environment to maintain absolute standards is sufficiently difficult to make it a viable technique. Holding the process environment constant over a period of hours, or even days, is much simpler.
This invention teaches the art of producing high accuracy phototools under short term constant environmental conditions, or of producing deliberately distorted phototools to compensate, for example, for different photoplotter and board manufacturing environmental conditions, or to match preexisting inaccurate mask sets, all without the necessity to recompute or corrupt the stored data.
Relating Photosignal to Position
The process of transferring optical data to or from an image can proceed continuously or discretely in time and space.
Raster scanning covers the image area in a series of nominally parallel stripes; each stripe forms one raster line. The data associated with a raster line image may be transferred to or from discrete blocks along that line or the data from contiguous blocks may be continuously blended; in either case, the block area of an image associated with a logically distinguished datum is called a pixel. Independently of the scanning technique employed, adjacent pixels may abut, may exhibit gaps between image areas covered or may overlap. In raster scanning, overlap characteristics along a raster line and across adjacent raster lines may vary either deliberately or inadvertently. Although pixel size may be changed for physically separated area of an image, this is not accomplished for contiguous image areas, at least in those cases where mechanical means are employed to establish the relationship between image data and position. Pixel data may be transferred serially pixel by pixel or a plurality of pixel data may be transferred simultaneously, typically for a plurality of adjacent raster lines.
Typical examples of raster scanning systems follow. Example One: the video display of a television set. Example Two: a drum plotter in which a photosensitive film is attached to the outer surface of a rapidly spinning cylindrical drum and an optical head is translated parallel to the drum axis in proximity to the drum surface. A control means provides for illuminating the drum pixel-by-pixel through the optical head with illumination of a strength appropriate to the pixel being illuminated; in this case, the raster lines lie circumferentially around the drum. Example Three: a typical photocopying machine in which a sensitized drum is rotated and a narrow line (a plurality of raster line pixels) illuminated simultaneously along the length of the drum.
A characteristic of raster scanning is that contiguous image areas are covered by the scanning process whether or not there is data to be transferred for some subset of that area, e.g., a television monitor ordinarily sweeps the entire screen area whether or not all areas are illuminated, and, having started to sweep a raster line, a typical drum scanner sweeps the entire raster line under the optical head whether or not individual pixel data is transferred.
Vector scanning covers image area selectively. Typical examples are X-Y plotters in which a pen is moved over paper to draw lines and X-Y photoplotters in which an optical head is moved by independently controlled lead screws over the surface of a photographic plate. Typically, the speed with which massive optical heads can be accurately translated in two dimensions is small compared, for example, to the surface speed of a revolving drum in a raster scanner. As a result, in vector scanning it is mechanically practical, and frequently required by throughput considerations, to vary the area exposed at any instant of time. For example, continuously exposing a large circular patch while moving the optical head exposes an image consisting of a wide line segment with semicircular ends; similarly exposing a smaller diameter patch would expose a narrower line segment. It is also possible, although rarely accomplished, to change the patch diameter as the optical head moves, resulting in a variable width line segment. Finally, individual patches can be rapidly exposed during translation of the optical head, or the head can be stopped to expose individual patches. A vector plotter, therefore, can be employed in a pixel oriented mode.
The teachings of this patent apply to the art of both vector and raster scanning. The teaching of the optimally chosen aperture shape applies to pixel oriented scanning.
In raster scanners, positioning of individual pixels is frequently accomplished by translating an optical head parallel to the axis of a rotating drum. Optical elements carried on the optical head are focussed on the surface of the image medium which is attached to the drum. Individual pixels frequently are defined by viewing, or exposing, the image medium through an aperture, which aperture is itself imaged by the optical system on the surface of the image medium. In plotting, the illuminating exposure is projected through the aperture onto the image surface and modulated to produce the desired exposure level. In scanning, the image medium is illuminated and light scattered by or transmitted through that medium is collected through the aperture for projection on a photosensitive detector surface. Thus, the image of the aperture defines the pixel boundaries.
Because a drum can be rotated smoothly and rapidly, it is possible, and desirable, to transmit data rapidly along the raster lines. Translation of the optical head, from one raster line to another is necessarily a much slower process; therefore, an oscillating mirror or rotating mirror or prism is frequently employed in place of a translated head. The preferred embodiment employs an optical head, but the teachings of this invention apply to any scanner system which images or reimages a physical aperture on an image surface or which forms an image equivalent to such an image.
Rotational motions can be easily controlled to obtain constant drum velocities over short terms; therefore, a timing system synchronized with the drum rotations may be employed to determine pixel position along a raster line. Typically, a shaft encoder or other source of positional information is employed to provide information as to the vertical position of the image medium relative to the focal spot projected from the optical head. The output of this encoder is typically a few thousand pulses per revolution; an electronic oscillator which outputs one pulse per pixel (typically a few pulses, say four, per shaft encoder pulse) is phase locked to the shaft encoder. The phase locked oscillator averages over several encoder pulses to smooth small irregularities in the encoder output. The oscillator output is the timing signal employed to control the positioning of pixels along raster lines; the timing signal frequency is typically in the order of one Megahertz. The spatial separation of pixels along the raster lines is determined by the product of three factors; the rotational velocity of the drum, the effective radius of the drum and image medium mounted thereon, and the time between the pixel timing pulses. Thus the underlying pixel grid is formed in the raster direction, and pixels can be transferred between image medium and data store at each pixel location.
Because this method of determining the position of the vertical pixel grid is independent of unwanted variations in the position of the focal spot generated by the optical head, artifacts in the imagery will be introduced, for example, by vibration of the optical head along the raster lines or by motion of the focussing elements with respect to the exposing light beam.
Most undesirable in the generation of printed circuit phototools are artifacts which cause separation between adjacent pixels; this may result in broken traces and failure of circuits properly to conduct electricity in the completed circuit board. A common, but undesirable, method to avoid pixel separation employs slightly oversize pixels. Five to 10% oversize pixels, which are spaced one raster grid element apart, overlap enough to mask these artifacts. However, narrow traces are appreciably too wide. Translation of the optical head parallel to the drum axis establishes the horizontal axis and grid. Typically, the head is coupled to a lead screw which is itself coupled to and driven by a stepper motor. Widely employed stepper motors step 400 steps per revolution; widely available high quality lead screws well matched to the physical characteristics and requirements of printed circuit plotters advance the carriage 1/10 inch per revolution. Therefore, four steps correspond to a displacement of the optical head of 1/1000 inch. Pixel grid spacing of about 1/1000 inch is typical of many optical scanners, including printed circuit phototool generators. Thus, transmitting from one to a few stepper motor control pulses to the stepper motor controller establishes the horizontal raster grid spacing.
Because the spatial positioning of the optical head is independent of the drum rotating beneath it, the same sort of artifacts can arise from unwanted motions as do in the vertical direction. In practice, because a massive head is translated more or less discontinuously, artifacts are far more pronounced in the horizontal than in the vertical direction. In addition, even systems driven by a very high quality lead screw may exhibit positional errors which are not acceptable for highest quality scanning. Assembly and mounting inaccuracies may introduce periodic once-per-revolution errors of many hundreds of millionths of an inch; this leads to the artifact known as banding. Inaccuracy of the lead screw pitch of ten millionths of an inch per lead, i.e., per revolution, may cumulate to more than 1/1000 inch per foot; this is an unacceptable error in high accuracy optical scanning.
One partial solution to horizontal positioning errors is to equip the scanner with a linear encoder employed to measure the actual position of the optical head with respect to a fixed base. This approach is very expensive, does not compensate for high frequency vibration-driven motions, and may leave unacceptable residual errors when employed in a high speed scanner. In any event, the most economical construction technique for high-quality high-speed scanners requires that the scanner structure be designed and build to be sufficiently stable that successive scans are accurately repeatable, even if not accurate in an absolute sense.
This invention teaches the art of high accuracy optical scanners whether or not equipped with permanently installed linear encoder systems.
Beam Forming Systems
There are many beam forming systems employed in the art of optical scanning. In plotting with aperture defined pixels, three criteria should be met: Economical operation, especially at high data rates, requires that the aperture be illuminated at the highest feasible level to maximize pixel exposure as a function of the illumination system employed. Exposure across the pixel must be sufficiently uniform for the resulting imagery to appear uniform under the appropriate criteria of judgment for the application. The imagery should be insensitive to the motions of the head and to positional errors of the light beam which result in the more undesirable image artifacts.
Conventional light sources, like arcs or filaments, produce widely divergent beams which require extensive condenser systems to illuminate the aperture. Also they are large, heavy and tend to be a source of excessive heat. Laser sources produce highly collimated beams capable of producing much larger exposures in the image plane. Both sources are most frequently mounted apart from the optical head, optical transfer systems are employed to deliver light to the optical head.
Because of its inherent divergence and lack of coherence, conventionally generated beams easily produce even illumination across the aperture; it is very difficult to make the illuminated patch on the aperture small enough efficiently to utilize the light; tight focussing of the input beam is required. Because of its relative lack of divergence and high coherence, it is difficult with a laser beam to produce an illuminated patch on the aperture which is large enough to avoid shading at the edges of the pixels and which avoids other coherence related illumination artifacts; very long focal length focussing systems or inverted afocal systems are employed.
In any case the head must be large to carry these optical systems, or compact telephoto designs must be employed, or systems split between the optical head and the underlying support structure must be employed. All these approaches are subject to the introduction of artifacts into the imagery. They are prone to misalignment and relative motion of their components, especially in comparison with the compact and simple apparatus taught herein.
Apertures
In scanning with a discrete pixel pattern, some edges of areas must be ragged because no pixel pattern permits perfectly smooth edges in all directions. To some extent smoothing can be obtained by scanning at very small grid spacing with very small pixels. If this spacing is small compared to the resolution of the scanner optics or, in plotting, to the resolution of the photosensitive image medium, apparently smooth edges result. Drawbacks here are that small grids imply large amounts of data to be handled, in some cases impossibly large amounts, relatively high data rates (and data transmission rates currently are one of the limits of the state-of-the-art), low throughput and for any given set of performance characteristics, higher cost than larger grid or pixel spacing. Smoothing can also be obtained by a number of techniques which reduce resolution. Drawbacks here lie in the inherent loss of resolution, loss of edge sharpness (which is not tolerable in line art reproduction), and in the associated costs.
In raster scanning an area is covered by a pattern of pixels each centered on a grid intersection. Usually the pixels are uniformly spaced in two dimensions, i.e. on a square grid. Customarily the pixels do not overlap significantly. Overlapping of oversize pixels is ordinarily considered to be a necessary evil, a defect in the scan pattern, engendered by the need to satisfy mechanical constraints imposed by less than perfect equipment, or by the necessity to fully cover the scanned area by a non-tessellating pattern. (A tessellating pattern covers a plane without overlap.) Coverage of the plane by repetitive pixel structures which do not tessellate the plane can be obtained by overlapping pixels in a manner which does not generate oversize images. The underlying grid spacing is simply adjusted to accomodate the pixel size; for example, by halving the grid spacing, pixels may be overlapped by 50% without increasing line widths. This coverage by overlapped pixels can provide fully or partially multiply-exposed coverage.
Unsual tessellations which are simple enough to be realized in practice use triangular, rectangular, or hexagonal pixels. The first of these requires alternating pixel orientation; the last requires staggered pixel placement. Both are impractical. Rectangular (actually square) pixels are ordinarily employed.
For full area average with non-overlapped pixels when edge quality is important for horizontal and vertical edges, the square pixel is excellent (and geometrically perfect). It is poor at other alignments, and worst at 45 degrees. In printed circuit artwork, 45 degree edges and traces are by far the most common orientations after horizontals and verticals, For any symmetrical tessellating pattern which gives good to perfect horizontal and vertical lines, symmetry requires 45 degree lines to have the worst edge characteristics. These statements are also valid for width limited areas, i.e. lines of finite width.
Of the non-tessellating pixel shapes, the circle provides edge raggedness independent of edge angle, but horizontal and vertical edges are, therefore, as ragged as any other edge. This is especially undesirable for printed circuit phototool generation.